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THE   TFLLODONTIA                                507

These animals have been regarded as ancestral [Rodents,, to
which the tooth characters just mentioned clearly show like-
nesses. The earliest known form is Esthonyx. This genus shows

FIG. 247. TUlotherium, fodie-ns.    Left lateral -view of skull.     (From Flower,
after Marsh.)
such primitive characters, compared with its later representatives*
as the existence of all three pairs of incisors ia the upper jaw,
but only two in the lower jaw. The enlarged incisors of both
jaws do not seem to have grown from persistent pulps.
A.7ichippodusf a later form, still preserves the upper pair of first
incisors in a vestigial form; the strong second incisors grew from
persistent pulps. The most recent genus, Tillotherium, shows, the
characteristics of the group at their height. The strong Bodent-
like, chisel-shaped incisors, which are reinforced by a small
additional pair in the upper jaws onlyy are persistent. The
grinding teeth are of the tritubercular pattern; there are three
of each kind in the upper jaw, but in the lower jaw only two
premolars on each side. This is at any rate the case with some,
while others have three. The canine, though present in both jaws,
is insignificant. As in many ancient types, there is an entepi-
condylar foramen in the humerus. The feet were five-toed, and
bore sharp, laterally-compressed claws. The skull has been com-
pared in general aspect to that of a Bear.